Soy foods are an excellent source of protein and can be a good option for meals without meat. Soy contains many phytochemicals, some of which have weak estrogen activity and seem to protect against hormone-dependent cancers in animal studies. Other compounds in soy have antioxidant or other helpful properties. There is a great deal of interest in the possible role of soy foods in reducing cancer risk in general and breast cancer in particular. But the evidence for such a role is mixed.
For the breast cancer survivor, current research finds no special benefits or harmful effects when no more than 3 servings of soy are eaten per day as part of a healthy diet. This compares to the amounts eaten in most Asian diets. But higher doses of soy may have estrogen-like effects, and higher levels of estrogens can cause certain breast cancers to grow and spread. For this reason, it is best for breast cancer survivors to avoid the high doses that are found in more concentrated sources such as soy powders and isoflavone supplements.